Microspheres are solid or hollow particles of size between 1 to 1000 micrometers. Hollow particles can have wall thickness from hundreds of microns to under 0.025 micron. Microspheres can be perfectly spherical, but are more often found as slightly ovoid or egg shaped, and can be appropriately described as being equiaxial in geometry. Microsphere materials are normally found as dry powders that have a strong density dependence on sphere outer and inner diameter. The true density of a hollow microsphere is lower than that of solid materials of the same composition. The density of hollow microspheres ranges from 95% to less than 1% of the parent bulk material density (as low as 0.025 g/cc or less). The spherical morphological shape is one of the unique features that differentiate these materials from others. A sphere has the lowest specific unit surface area of any geometric form and has a high realizable packing density. In bulk form, microspheres can behave in fluid like manner, deforming semi-elastically without resistance to applied stresses and can roll past one another like ball bearings, with no rough surfaces or branches to entangle. When dispersed as slurry or paint, at common loadings, they act to extend the volume and enhance drying and adhesion properties of the dispersion medium, with minimal impact on its original viscosity. Materials design engineers and others skilled in the art often use microspheres to increase the solid content of coating solutions while having the ability to maintain appropriate coating application and flow characteristics. Higher solids loadings in various applications can reduce volatile organic compound concentrations (VOCs), shrinkage, and drying time in paints. The large volume that microspheres displace for a given weight is an important attribute in many applications. Because hollow spheres will tend to lower the density of materials they are added to paint or coating formulations. A low-density coating or paint formulation will atomize better, give less spatter when rolling, and sag less once applied and since a small weight-addition of microspheres increases the batch volume significantly, formulation cost can be reduced.
Since microspheres are closed-cell, gas-filled or hollow particles, they are extremely good insulators. Thermal and acoustic insulation properties of coatings or substrates can be improved by the addition of microspheres. Heat-insulation properties of the ceramic microspheres are directly related to their thermal conductivity and radiation. The role of radiation increases with increase in temperature and becomes prominent effect in thermal insulation above ˜700° C. The thermal conductivity of the hollow spheres depends on the shell material and the low conducting gas inside the spheres. In general, the lower the thermal conductivities of the wall material and the internal gas (or vacuum), the lower the effective shell thermal conductivity. Heat insulation properties are also defined by special features of emissivity and scattering of thermal radiation by thin-walled hollow particles. Glass or polymeric hollow spheres used in thermal insulation applications need an overcoating of a high emissivity material to improve the heat-insulation properties.
Microspheres are widely used in the fiber-reinforced polyester industry to improve the manufacturing process of shower stalls and boats. Lighter, more-durable fiberglass products are a direct result of the creative use of microspheres. Thick-film ink, mining explosives, and rubber and plastic products of all descriptions are just a few other examples of the many products that are made better with these versatile materials. The benefits derived by these diverse end uses vary—some are unique to a specific industry, while others are common goals shared by many manufacturers.
Synthesis of ceramic microspheres include soda glass, aluminum silicate, silicon dioxide, aluminum phosphate, calcium phosphate, calcium silicate and titanium oxide etc. [J. Szepvolgy, Z. Karoly, Preparation of Hollow Alumina Microspheres by RF Thermal Plasma, Key Engineering Materials Vols. 264-268, 101-104 (2004); U.S. Pat. No. 6,110,528; J. K. Cochran, Ceramic hollow spheres and their applications, Current Opinion in Solid State & Materials Science, 3, 474-479 (1998)]. The prior art aluminum phosphate microspheres are amorphous and highly porous suitable for catalyst supports.
Hollow ceramic spheres can be prepared by several processing methods. Melting of the components in flame and foam using foaming agents like sulfur. This method leads to very large spheres 70-100 microns. Silica hollow spheres can be formed using polymer spheres as templates and high temperature annealing leads to hollow spheres after burning out organic polymers. Titanium oxide microspheres are formed by interface-assembly synthesis. (Nakashima T, Kimizuka N. J Am Chem Soc. 2003 May 28; 125(21):6386-7). Metallic spheres (for example, syntactic foams) are also known in prior art. Recently spray drying method is being utilized in making ceramic microspheres [E. Sizgek, J. R. Bartlett, and M. P. Brungs, Production of Titanate Microspheres by Sol-Gel and Spray-Drying, Journal of Sol-Gel Science and Technology, Vol. 13, pp. 1011-1016 (1998); P. Luo and T. G. Nieh, preparation hydroxyapatite powders with controlled morphology, Biomaterials, Vol. 17, pp. 1959-1964, (1996)]. Glass microspheres possess alkali metals which diffuse during processing and in field. So need additional protection layers to prevent the alkali metal leaching. These ions are also detrimental to the electrical properties of glass. Presence of boron in the precursor solution affects the stability of solutions resulting in sol formation or precipitation before undergoing heat treatment to form microspheres.
The limitations of the prior art microspheres include a) lack of morphological and thermal stability at elevated temperatures, b) lack of black or grey or other colored materials useful for various applications including pigments and paints, c) lack of nanocomposite architectures to exploit multifunctional properties, and d) lack of high emissivity useful for thermal insulation applications. The present invention overcomes the aforementioned limitations and, in addition, offers a low-cost and processing approach to synthesize both solid and hollow microspheres.